In the design of aircraft wings, one objective is to provide a wing that has the characteristic of a gradual stall progression. It is also desirable that adverse pitching moment characteristics be avoided so that a safe recovery can be easily made from a stall condition. These design criteria are particularly critical for swept wing aircraft with high lift devices, such as are used on modern high speed passenger airplanes.
In swept wing aircraft, the outboard wing sections are behind the center of lift of the aircraft, whereas the inboard portion of the wings are forward of the center of lift. One noticeable disadvantage of most swept wing aircraft is that during stall conditions, the outboard portion of the wing stalls prior to the inboard portion. Thus, as lift is lost on the outboard wing portions, the center of lift moves inward and forward. This forward movement of the center of lift results in the undesirable tendency for the aircraft nose to pitch up, thereby accelerating stall conditions on the remainder of the wing. Ultimately, as a stall condition develops on the remainder of the wing areas, lift is lost and the adverse pitching moment is eliminated. Then, under most conditions, the aircraft nose will fall downwardly and recovery from a stall can be performed.
Recovery from a stall having accompanying nose-up pitching moment is further aggravated in aircraft which employ horizontal stabilizers mounted on the top of a vertical stabilizer, i.e., "T-tail" configurations. At maximum angles of attack in such aircraft, the shed wake of the stalling wing and of any rear mounted engine nacelles flows rearward to envelop the tail assembly. This shed wake adversely affects the horizontal stabilizer effectiveness. The resulting reduction in tail effectiveness in combination with an inherent nose-up pitching moment in a stalling wing exacerbates recovery from a stall. Where a deep stall phenomenon is experienced, the aircraft can pitch nose-up upon stall and then drops more vertically at a relatively high angle of attack, in which case it may be more difficult for the pilot to bring the aircraft back to the desired angle of attack and with appropriate control of the aircraft.
Unlike the situation just described where adverse nose-up pitching moment is experienced, desirable stall characteristics in an aircraft wing are provided when a moderate nose down pitching moment, without roll, occurs when a wing experiences a stall condition. Generally, it is desirable that the onset of a stall condition occur with changes in lift patterns that avoid abrupt changes in pitching moments. In particular, it is desirable to eliminate the onset of an adverse nose-up pitching moment when loss of lift is experienced in swept wing aircraft.
One approach that has long been used by aircraft designers in an attempt to minimize the above-mentioned adverse nose-up pitching moment characteristics is the use of wing twist. That is, the inboard wing sections are provided with a greater angle of attack than the outboard wing sections. Theoretically, the inboard wing sections will therefore tend to experience boundary layer separation and thus see stall conditions earlier than the outboard wing sections which are positioned at a lower angle of attack with respect to the relative wind. However, in swept wing aircraft, a spanwise moving boundary layer, flowing from the inboard wing section toward the outboard wing section, tends to increase the boundary layer thickness on the outboard wing section. This phenomenon decreases the lifting capability of the outboard wing sections and makes them more prone to stalling than the inboard sections. Considerable attention has been directed toward devices which deal with improved control of boundary layer flows; those devices of which we are aware will be discussed herein below.
Other attempts to solve the problem of adverse nose-up pitching moment during stall have included: (a) the application of leading edge fences, (b) the use of slot-gap cover plates, and (c) the use of stall strips at various locations.
Leading edge fences have proven somewhat effective in controlling adverse post-stall characteristics, and are currently employed on numerous swept wing aircraft. The devices have been most extensively utilized on swept wing aircraft having aft-body mounted engines and "T-tails". The leading edge fences are most effective for aircraft cruise configurations; they are not particularly effective for takeoff or landing. Another disadvantage of the leading edge fence devices is that they increase drag during cruise.
Use of slot-gap cover plate has also been suggested as a way to improve pitching characteristics at stall during high angles of attack. Such a plate seals the space between a main wing section and a leading edge device, and typically would be employed on inboard wing sections. One aerodynamic disadvantage of such a device is the large wake that such a device sheds, with a corresponding loss in tail effectiveness for "T-tail" aircraft. Thus, the hazard of a "deep stall" phenomenon, as mentioned above, may be encountered. Also, a significant loss of maximum lift capability results when this device is used. Another disadvantage is the apparent need to make the cover plate movable either in conjunction with deployment of the leading edge device, or as a separately controlled device.
Stall strips are used on many types of aircraft to control the stall pattern and/or stall progression. Stall strips can be located in either an exposed position, such as on a wing leading edge near the wing root, or in a concealed position, such as on a main wing section that is concealed during cruise by a movable leading edge device. When stall strips are located in concealed positions, they do not impose a drag penalty during cruise. However, stall strips have several undesirable features, including:
(a) significant loss in maximum coefficient of lift, and
(b) creation of a large wake locally along the wing span. The latter item is particularly undesirable, since an increased shed wake adversely affects "T-tail" performance, with the result that improved stability associated with localized inboard lift loss is offset by a corresponding loss of control effectiveness.
A search of the patent literature has disclosed a number of patents which have, in part, attempted to solve the aforementioned problems of stall stability in swept wing aircraft, as follows:
U.S. Pat. No. 2,041,793, issued May 26, 1936 to Stalker, discloses slotted wings which include vortex generating devices in the slots which energize the air and increase lift from air passing through the slot. The vortex generating vanes also serve to mix the boundary layer air, to decrease boundary layer drag.
U.S. Pat. No. 2,070,006, issued Feb. 9, 1937 to Eaton, Jr. et al., discloses a spoiler system for use in aircraft having leading edge slats. The spoilers are located at the forward portion of the fixed wing section, so that spoilers are hidden and therefore inoperative when the slats are not deployed.
U.S. Pat. No. 2,743,888, issued May 1, 1956 to Lippisch, shows a swept wing aircraft with pivoting winglets along each of the leading edges. The winglets are pivoted outwardly and forwardly at low speed to create a series of discontinuities along the wing leading edge, to reduce drag and to increase lift.
U.S. Pat. No. 2,769,602, issued Nov. 6, 1956 to Furlong, shows a chord extension device which is adapted to fit smoothly along the leading portion of a swept wing to increase the chord along a segment of such wings. Also, Furlong discloses that the chord extensions provide discontinuities along the leading edge which determine the location of initial boundary layer separation, thus providing acceptable stability variations throughout the entire lift range.
U.S. Pat. No. 2,793,826, issued May 28, 1957 to Fiedler, shows the use of a split wing to overcome low speed wing tip stall on a swept wing aircraft. The Fiedler invention utilizes a matching wing pair, wherein one section is rigidly mounted in a standard sweptback configuration, and a second portion is hinged at the wing root and extends forward and outward for improved low speed stability.
U.S. Pat. No. 3,370,810, issued Feb. 27, 1968 to Shevell et al., illustrates the use of an underwing aerodynamic body to create a vortex which sweeps across the upper wing surface in a direction opposite to the boundary layer flow toward the wing tip. Shevell discloses that a vortex thus created is of relatively high strength and adequate to prevent wing tip stall.
U.S. Pat. No. 3,523,661, issued Aug. 11, 1970 to Rethorst, illustrates use of a series of asymmetric underwing diffusers to reduce spanwise air flow and recover energy therefrom.
U.S. Pat. No. 3,638,886, issued Feb. 1, 1972 to Zimmer, illustrates a leading edge slotted flap configuration which extends to reveal a uniformly sized fixed main wing portion.
U.S. Pat. No. 4,032,087, issued June 28, 1977 to Cleaves, discloses a leading edge spoiler system, and includes a series of air vents/spoiler bearing supports.
U.S. Pat. No. 4,293,110, issued Oct. 6, 1981 to Middleton et al., illustrates a series of leading edge vortex flaps for use on highly swept back aircraft. In an extended position, each of the flaps forms a discontinuous outboard end which is useful in vortex shedding to reduce spanwise airflow. When stowed for cruise, the flaps form a continuous smooth leading edge.
U.S. Pat. No. 4,323,209, issued Apr. 6, 1982 to Thompson, illustrates the use of a series of forwardly projecting fingers to shed a vortices rearward across an airfoil.
U.S. Pat. No. 4,378,922, issued Apr. 5, 1983 to Pierce, illustrates use of fuselage strakes in a low aspect ratio aircraft. These strakes, however, are directed at improving lateral stability at high angles of attack, rather than for changing aircraft stall characteristics.
U.S. Pat. No. 4,553,721, issued Nov. 19, 1985 to Jorgensen, discloses a spoiler device for use on an aircraft having a leading edge slat. The spoiler is designed to increase drag and reduce lift on the inboard portions of swept wings, so as to prevent a forward migration of the center of lift, with resultant adverse nose-up pitching moment.
Finally, prior U.S. Pat. No. 4,702,441, issued Oct. 27, 1987 to Wang, discloses use of stall strips of either rigid or spring mounted construction. The stall strips are exposed when the slats are deployed. They produce an air flow disturbance between a fixed main wing section and an extensible leading edge slat, to promote stall formation over the inboard wing portion.